My husband just vacuumed the pool 2 days ago and today I can see aprox. 25% of the pool liner is covered in yellowis;/green algee again!

We have treated with an algeecide twice in the past 2 weeks.

The pool is a 17,200 gal above ground pool with vinal liner.
I did the pool kit test this morning and the following results were obtained:
cyanuric acid 80 ppm
total CL 4.2
Free CL 4.0
ph 7.0 - though I know that this may not be correct since the test kit said it should not be done if CL was over 3.0

Do I need to shock it? Treat with algeecide again?
Please hlep, I'm tired of being a slave to my pool.

First, you are always a slave to your pool. Some of us are just happy slaves.

Second, you have high CYA which means you need higher levels of chlorine in order to give the algae the "shock and awe" treatment...which is what it needs. Algaecide is more of a preventative than a cure.

Unstabilized Chlorine is your friend. Go to Walmart and buy 5 gallons of ultra-bleach (chem geek check my numbers!). This should bring your chlorine up to 15ppm where you need to stay for 24 hours. During this time you need to brush/brush/brush with your pool brush on a pole. to make sure to expose all the algae to the chlorine. You will also need to run your filter and pump 24/7 as you will have a lot of dead algae floating about.

You should also bring down your CYA level which is done through dilution (draining a foot or two of water and re-filling with fresh).

Stop using pucks or any form of stabilized chlorine or your resistance will only increase.

At a CYA level of 80 ppm, you would have had to keep your Free Chlorine (FC) at an absolute minimum of 7.5% of the CYA level or 6.0 ppm with the usual target being 11% of the CYA level or 8.8 ppm. To shock the pool to get rid of the algae will take about 40% of the CYA level or 32 ppm FC, but with a vinyl pool you may not want to go that high and instead just maintain 20 ppm FC which means checking the level several times a day and adding more chlorine (the algae will consume a lot of chlorine as it gets killed).

Algae can most definitely be killed with chlorine alone as seen here . It takes patience and persistence. You need to get your CYA lower at some point, but after you've gotten rid of the algae you need to either keep the minimum FC level I indicated above or you need to add a weekly maintenance dose of PolyQuat 60 algaecide every week (which can get expensive). Lowering your CYA level will let you keep a lower FC level to keep away algae.